Polarizing devices, including polarizers and polarizing beam-splitters (PBS), are essential optical components and are currently widely used in optical instruments, lasers, electro-optic displays, optical recording, etc. In polarizers, only the transmitted or reflected light is used, the other beam is of no essence. In PBSs, both the transmitted and the reflected beams are utilized and are equally important. Several parameters are often used to describe the performance of a polarizer or PBS: 1) the wavelength range over which the polarizer or PBS is operating; 2) the angular field of the incident light in which the polarizer or PBS is effective; 3) the extinction ratio of the desired to the unwanted a polarized light after the light passes through or is reflected by the polarizer or PBS; and 4) the transmittance or reflectance for the desired polarization.
Commonly available polarizers and polarizing beam-splitters can be divided into several types that depend upon different physical principles: pile-of-plates polarizers, reflection polarizers, Polaroid sheet polarizers, polarizers based on birefringent crystals, metallic grid polarizers, and thin film interference polarizers or PBSs.
Since the present invention is related to a thin film polarizing device, the prior art in this field is reviewed in the following section.
Thin film polarizers or PBSs are based on the light interference in thin films, sometimes also in combination with other physical phenomena. Conventional thin film polarizers or PBSs are relatively versatile in terms of design and fabrication; they are not limited by size and they can be manufactured on large scale and at a low cost. However, they are also limited in performance.
The most commonly used wide band thin film polarizer was invented in 1946 by MacNeille (U.S. Pat. No. 2,403,731). It is based on the Brewster angle phenomenon and on light interference in thin films. Because of its importance, the theory on which it is based and its performance will be compared at some length in Section 6 with those of the PBS operating at angles greater than the critical angle proposed in this paper. Here it is sufficient to say that the MacNeille polarizer operates over a broad band of wavelengths, but is very sensitive to the angle of incidence. Once the incident angle moves away from the Brewster angle by .+-.2.degree., the performance of the polarizer deteriorates dramatically. The device can be used as a PBS, but the extinction ratio for the reflected beam in the conventional configuration of the device is rather low.
Another thin film PBS is based on the separation that occurs between the edges for s- and p-polarized light of a cut-off filter that has been deposited onto a parallel plate or prism cubes and that is illuminated at an oblique angle. In the region between the two edges, s-polarized light is reflected and p-polarized light is transmitted. The angular field of a plate polarizer is relatively large compared to that of a MacNeille polarizer. The extinction ratio for the transmitted beam can be high if a large number of layers is used to reflect the s-polarized light. However, it is harder to achieve a high rejection ratio for the reflected beam. The plate polarizer has a very small bandwidth. It is often used in laser systems.
For many applications, there is a need for low-cost and easily-producible non-absorbing, broadband, wide-angle polarizing beam-splitters (PBS). For example, a high efficiency back-lighting system for direct-view LCDs, disclosed in our co-pending patent application derived from US provisional application no. 60/110,166, requires the use of non-absorbing PBS to recover the polarization loss. The light loss in current LCDs is more than 50%. However, none of the currently available polarizers or PBS meet the display requirements either because of their high absorption, or limited bandwidth, small angular field, limited size or high cost.
The only thin film polarizing beam-splitter that meets the display requirements is the novel PBS that was described in the U.S. Pat. No. 5,912,762 by Li Li and J. A. Dobrowolski. This PBS is broad-band, wide-angle, has high extinction ratios for both transmitted and reflected beams. It is based on the effects of frustrated total internal reflection and light interference in thin films. It works at angles larger than the critical angle. The refractive index of the substrate has to be larger than that of the low-index material. The higher the refractive index of the substrate, the better the performance is.
The above patent describes a new concept of designing polarizing beam-splitters by using the effects of frustrated total internal reflection (FTIR) and interference. The PBS consists of low and high refractive index layers. The refractive indices of the low or high index layers are lower or higher than that of the substrate, respectively. The incident angle upon the low index layers is larger than the critical angle. As a result, the admittances of these low index layers have only imaginary parts. Therefore, they behave like perfect metals and they only attenuate light but do not absorb light. If these low index layers are thin, then frustrated total internal reflection will occur inside them, therefore, we can also call them FTIR layers. By combining the low and high index layers in a symmetrical structure and by carefully choosing the right layer thicknesses, the equivalent admittance of the symmetrical structure can have very different values for s-and p-polarized light over a range of angles and wavelength. Therefore, the symmetrical structure can be used to design very broad-band and wide-angle polarizing beam-splitters.
Although the above-mentioned PBS has much better performance than traditional devices, it has some disadvantages for direct-view LCD back-lighting systems. In these systems, plastic substrates are preferred because low cost and low weight are essential factors in this application. Unfortunately, the highest refractive of indices of optical plastics is about 1.60, corresponding to that of polycarbonate, lower than that of some high-index glasses (for example 1.75). Therefore, the performance of the PBS is less satisfactory. In addition, the PBS requires accurate thickness control that is not desirable for large-scale production. Furthermore, for some cases, the PBS requires optical contacts in cementing two substrates together. This may limit the size of the PBS and also result in a high manufacturing cost.
An object of the invention is to address this problem.